Interpretive Summary: Soil borne arbuscular mycorrhizal (AM) fungi form a symbiotic (mutualistic) relationship with most plants. This relationship is vital to plant productivity in a low input sustainable agriculture (LISA) regime since these fungi provide mineral nutrition, disease and water stress tolerance to plants while optimizing the conservation of soil and water resources. Over this past century, excessive cultivation with pesticides and synthetic fertilizers has decreased the activity and efficacy of native communities of AM fungi. Our aim is to efficiently produce large quantities of AM fungi for soil inoculation to ease the transition from high input to LISA. It is not practical to produce large quantities of inoculum at this time since AM fungi are obligate symbionts that are unable to complete their life cycle in the absence of a host plant. We are studying the metabolism and genetics that underlie this symbiosis in order to establish axenic culture, (fungus production in the absence of the host plant). In this study we determined 1) what critical nutrients are provided by the host plant during the symbiosis, 2) when and in what form they are transferred to the fungus and 3) how they are used by the fungus. This information is essential for formulating strategies for the axenic production of AM fungal inoculum.

Technical Abstract:
Arbuscular mycorrhizal (AM) fungi take up photosynthetically fixed carbon from plant roots and translocate it to their external mycelium. Previous experiments have shown that fungal lipid synthesized from carbohydrate in the root is one form of exported carbon. In this study an analysis of the labeling in storage and structural carbohydrates after 13C1 glucose was provided to AM roots shows that this is not the only pathway for the flow of carbon from the intraradical to the extraradical mycelium. Labeling patterns in glycogen, chitin and trehalose during the development of the symbiosis are consistent with a significant flux of exported glycogen. The identification, among expressed genes, of putative sequences for glycogen synthase, glycogen branching enzyme, chitin synthase and for the first enzyme in chitin synthesis (GFAT) is reported. The results of quantifying of glycogen synthase gene expression within mycorrhizal roots, germinating spores and extraradical mycelium are consistent with labeling observations using 13C labeled acetate and glycerol, both of which indicate that glycogen is synthesized by the fungus in both germinating spores and during symbiosis. Implications of the labeling analyses and gene sequences for the regulation of carbohydrate metabolism are discussed, and a fourfold role for glycogen in the AM symbiosis is proposed: sequestration of hexose taken from the host, long term storage in spores, translocation from intraradical to extraradical mycelium and buffering of intracellular hexose levels throughout the life cycle.